Adjusting Polishing Rates by Using Spectrographic Monitoring of a Substrate During Processing

ABSTRACT

A computer-implemented method includes receiving a sequence of current spectra of reflected light from a substrate; comparing each current spectrum from the sequence of current spectra to a plurality of reference spectra from a reference spectra library to generate a sequence of best-match reference spectra; determining a goodness of fit for the sequence of best-match reference spectra; and determining at least one of whether to adjust a polishing rate or an adjustment for the polishing rate, based on the goodness of fit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/094,379, filed on Sep. 4, 2008.

BACKGROUND

The present invention relates generally to spectrographic monitoring ofa substrate during chemical mechanical polishing.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the conductive layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is typically placed against a rotating polishing diskpad or belt pad. The polishing pad can be either a standard pad or afixed abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing liquid, suchas a slurry with abrasive particles, is typically supplied to thesurface of the polishing pad.

One problem in CMP is using an appropriate polishing rate to achieve adesirable wafer profile, e.g., a wafer that includes a substrate layerthat has been planarized to a desired flatness or thickness, or adesired amount of material has been removed. Overpolishing (removing toomuch) of a conductive layer or film leads to increased circuitresistance. On the other hand, underpolishing (removing too little) of aconductive layer leads to electrical shorting. Variations in the initialthickness of the substrate layer, the slurry composition, the polishingpad condition, the relative speed between the polishing pad and thesubstrate, and the load on the substrate can cause variations in thematerial removal rate. More generally, an actual polishing rate can bedifferent from an expected polishing rate. Therefore, the polishingendpoint cannot be determined merely as a function of the polishingtime.

SUMMARY

In general, one aspect of the subject matter described in thisspecification can be embodied in a computer-implemented method thatincludes receiving a sequence of current spectra of reflected light froma substrate. Each current spectrum from the sequence of current spectracan be compared to a plurality of reference spectra from a referencespectra library to generate a sequence of best-match reference spectra.A goodness of fit can be determined for the sequence of best-matchreference spectra, and at least one of whether to adjust a polishingrate or an adjustment for the polishing rate can be determined, based onthe goodness of fit. Other embodiments of this aspect includecorresponding systems, apparatus, and computer program products.

These and other embodiments can optionally include one or more of thefollowing features. The goodness of fit can be less than or equal to athreshold value and the polishing rate can be adjusted. The goodness offit can be greater than a threshold value and the polishing rate can bemaintained. The method can further include modifying a pressure in acarrier head using the adjustment for the polishing rate.

The sequence of current spectra can include spectra from a plurality ofradial zones on the substrate, and generating a sequence of best-matchreference spectra can include generating a plurality of sequences ofbest-match reference spectra, each zone having an associated sequence ofbest-match reference spectra. Determining an adjustment can includecalculating a polishing rate for each particular zone based on thebest-match reference spectra from the sequence of current spectraassociated with the particular zone.

In addition, determining an adjustment can include calculating amodified polishing rate for at least one zone such that the entiresubstrate reaches a target thickness at approximately the same time.Determining an adjustment can further include calculating a modifiedpolishing parameter to provide the modified polishing rate, and themethod can further include adjusting the polishing rate by modifying thepolishing parameter.

Determining the goodness of fit can include determining a sequence ofindices from the reference spectra library that corresponds to thesequence of best-match reference spectra, and fitting the sequence ofindices to a function. The function can produce a line. The goodness offit can be based on an amount that the sequence of indices varies fromthe function. The method can further include calculating the amountusing a sum of squared differences between the sequence of indices andthe function. The method can further include determining a polishingendpoint based on the sequence of best-match reference spectra and thegoodness of fit.

In general, another aspect of the subject matter described in thisspecification can be embodied in a computer-implemented method thatincludes receiving a sequence of current spectra of reflected light froma substrate. Each current spectrum from the sequence of current spectracan be compared to a first plurality of reference spectra from a firstreference spectra library to generate a first sequence of best-matchreference spectra. Each current spectrum from the sequence of currentspectra can be compared to a second plurality of reference spectra froma second reference spectra library to generate a second sequence ofbest-match reference spectra. A first goodness of fit for the firstsequence of best-match reference spectra and a second goodness of fitfor the second sequence of best-match reference spectra can bedetermined. At least one of whether to adjust a polishing rate or anadjustment for the polishing rate can be determined, based on at leastone of the first goodness of fit and the second goodness of fit. Otherembodiments of this aspect include corresponding systems, apparatus, andcomputer program products.

These and other embodiments can optionally include one or more of thefollowing features. At least one of the first goodness of fit and thesecond goodness of fit can be less than or equal to a threshold value,and the polishing rate can be adjusted. Determining the first goodnessof fit can include determining a first sequence of indices from thefirst reference spectra library that corresponds to the first sequenceof best-match reference spectra, and fitting the first sequence ofindices to a first function. Determining the second goodness of fit caninclude determining a second sequence of indices from the secondreference spectra library that corresponds to the second sequence ofbest-match reference spectra, and fitting the second sequence of indicesto a second function.

The first function and the second function each can produce a line. Thefirst goodness of fit can be based on a first amount that the firstsequence of indices varies from the first function, and the secondgoodness of fit can be based on a second amount that the second sequenceof indices varies from the second function. The method can furtherinclude calculating the first amount using a sum of squared differencesbetween the first sequence of indices and the first function, andcalculating the second amount using a sum of squared differences betweenthe second sequence of indices and the second function. The method canfurther include determining a polishing endpoint based on at least oneof the first sequence of best-match reference spectra and the firstgoodness of fit, and the second sequence of best-match reference spectraand the second goodness of fit.

In general, another aspect of the subject matter described in thisspecification can be embodied in a controller that includes a comparisonmodule that compares each current spectrum from a sequence of currentspectra of reflected light from a substrate to a plurality of referencespectra from one or more reference spectra libraries to generate asequence of best-match reference spectra, and determines a goodness offit for the sequence of best-match reference spectra. The controller canalso include an adjustment module that determines at least one ofwhether to adjust a polishing rate or an adjustment for the polishingrate, based on the goodness of fit. Other embodiments of this aspectinclude corresponding systems, methods, and computer program products.

In general, another aspect of the subject matter described in thisspecification can be embodied in a system that includes a light source,a detector that detects light reflected from the substrate anddetermines a sequence of current spectra from the light, and acontroller. The controller can include a comparison module that compareseach current spectrum from the sequence of current spectra to aplurality of reference spectra from one or more reference spectralibraries to generate a sequence of best-match reference spectra, anddetermines a goodness of fit for the sequence of best-match referencespectra; and an adjustment module that determines at least one ofwhether to adjust a polishing rate or an adjustment for the polishingrate, based on the goodness of fit. Other embodiments of this aspectinclude corresponding methods, apparatus, and computer program products.

As used in the instant specification, the term substrate can include,for example, a product substrate (e.g., which can include multiplememory or processor dies), a test substrate, a bare substrate, and agating substrate. The substrate can be at various stages of integratedcircuit fabrication, e.g., the substrate can be a bare wafer, or it caninclude one or more deposited and/or patterned layers. The termsubstrate can include circular disks and rectangular sheets.

Possible advantages of implementations of the invention can include thefollowing. The polishing process can be less sensitive to noise,disturbances, and other variations, and thus the reliability of thepolishing process using an appropriate polishing rate can be improved.As a result, the accuracy and/or precision of polishing an individualwafer to a desired thickness and wafer to wafer uniformity can beimproved.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,aspects, and advantages of the invention will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a polishing apparatus.

FIG. 2 is an overhead view of an example rotating platen and illustrateslocations where in situ measurements can be taken.

FIG. 3 illustrates an example index trace from a spectrographicmonitoring system showing a good data fit.

FIG. 4 illustrates an example index trace from a spectrographicmonitoring system showing a poorer data fit.

FIG. 5 is a flow diagram of an example process for adjusting a polishingrate.

FIG. 6 is a flow diagram of another example process for adjusting apolishing rate.

FIG. 7 is a flow diagram of an example process for adjusting thepolishing rate in zones to achieve a desired profile.

FIG. 8 illustrates an example graph of polishing progress versus timefor a process in which the polishing rates are adjusted.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a polishing apparatus 20. The polishingapparatus 20 includes a rotatable disk-shaped platen 24, on which apolishing pad 30 is situated. The platen is operable to rotate about anaxis 25. For example, a motor can turn a drive shaft 22 to rotate theplaten 24.

An optical access 36 through the polishing pad is provided by includingan aperture (i.e., a hole that runs through the pad) or a solid window.The solid window can be secured to the polishing pad, although in someimplementations the solid window can be supported on the platen 24 andproject into an aperture in the polishing pad. The polishing pad 30 isusually placed on the platen 24 so that the aperture or window overliesan optical head 53 situated in a recess 26 of the platen 24. The opticalhead 53 consequently has optical access through the aperture or windowto a substrate being polished. The optical head is further describedbelow.

The polishing apparatus 20 includes a combined slurry/rinse arm 39.During polishing, the arm 39 is operable to dispense a polishing liquid38, such as a slurry. Alternatively, the polishing apparatus includes aslurry port operable to dispense slurry onto the polishing pad 30.

The polishing apparatus 20 includes a carrier head 70 operable to holdthe substrate 10 against the polishing pad 30. The carrier head 70 issuspended from a support structure 72, for example, a carousel, and isconnected by a carrier drive shaft 74 to a carrier head rotation motor76 so that the carrier head can rotate about an axis 71. In addition,the carrier head 70 can oscillate laterally in a radial slot formed inthe support structure 72. In operation, the platen is rotated about itscentral axis 25, and the carrier head is rotated about its central axis71 and translated laterally across the top surface of the polishing pad.

The polishing apparatus also includes an optical monitoring system,which can be used to determine whether to adjust a polishing rate or anadjustment for the polishing rate as discussed below. The opticalmonitoring system includes a light source 51 and a light detector 52.Light passes from the light source 51, through the optical access 36 inthe polishing pad 30, impinges and is reflected from the substrate 10back through the optical access 36, and travels to the light detector52.

A bifurcated optical cable 54 can be used to transmit the light from thelight source 51 to the optical access 36 and back from the opticalaccess 36 to the light detector 52. The bifurcated optical cable 54 caninclude a “trunk” 55 and two “branches” 56 and 58.

As mentioned above, the platen 24 includes the recess 26, in which theoptical head 53 is situated. The optical head 53 holds one end of thetrunk 55 of the bifurcated fiber cable 54, which is configured to conveylight to and from a substrate surface being polished. The optical head53 can include one or more lenses or a window overlying the end of thebifurcated fiber cable 54. Alternatively, the optical head 53 can merelyhold the end of the trunk 55 adjacent the solid window in the polishingpad.

The platen includes a removable in-situ monitoring module 50. Thein-situ monitoring module 50 can include one or more of the following:the light source 51, the light detector 52, and circuitry for sendingand receiving signals to and from the light source 51 and light detector52. For example, the output of the detector 52 can be a digitalelectronic signal that passes through a rotary coupler, e.g., a slipring, in the drive shaft 22 to a controller 60, such as a computer, forthe optical monitoring system. Similarly, the light source can be turnedon or off in response to control commands in digital electronic signalsthat pass from the controller through the rotary coupler to the module50.

The in-situ monitoring module can also hold the respective ends of thebranch portions 56 and 58 of the bifurcated optical fiber 54. The lightsource is operable to transmit light, which is conveyed through thebranch 56 and out the end of the trunk 55 located in the optical head53, and which impinges on a substrate being polished. Light reflectedfrom the substrate is received at the end of the trunk 55 located in theoptical head 53 and conveyed through the branch 58 to the light detector52.

The light source 51 is operable to emit white light. In oneimplementation, the white light emitted includes light havingwavelengths of 200-800 nanometers. A suitable light source is a xenonlamp or a xenon mercury lamp.

The light detector 52 can be a spectrometer. A spectrometer is basicallyan optical instrument for measuring intensity of light over a portion ofthe electromagnetic spectrum. A suitable spectrometer is a gratingspectrometer. Typical output for a spectrometer is the intensity of thelight as a function of wavelength (or frequency).

The light source 51 and light detector 52 are connected to a computingdevice, e.g., the controller 60, operable to control their operation andto receive their signals. The computing device can include amicroprocessor situated near the polishing apparatus, e.g., a personalcomputer. With respect to control, the computing device can, forexample, synchronize activation of the light source 51 with the rotationof the platen 24.

As shown in FIG. 2, as the platen rotates, the computing device cancause the light source 51 to emit a series of flashes starting justbefore and ending just after the substrate 10 passes over the in-situmonitoring module (each of points 201-211 depicted represents a locationwhere light from the in-situ monitoring module impinged and reflectedoff.) Alternatively, the computing device can cause the light source 51to emit light continuously starting just before and ending just afterthe substrate 10 passes over the in-situ monitoring module. In eithercase, the signal from the detector can be integrated over a samplingperiod to generate spectra measurements at a sampling frequency.Although not shown, each time the substrate 10 passes over themonitoring module, the alignment of the substrate with the monitoringmodule can be different than in the previous pass. Over one rotation ofthe platen, spectra are obtained from different radii on the substrate.That is, some spectra are obtained from locations closer to the centerof the substrate and some are closer to the edge. In addition, overmultiple rotations of the platen, a sequence of spectra can be obtainedover time.

In operation, the computing device can receive, for example, a signalthat carries information describing a spectrum of the light received bythe light detector 52 for a particular flash of the light source or timeframe of the detector. Thus, this spectrum is a spectrum measuredin-situ during polishing.

Without being limited to any particular theory, the spectrum of lightreflected from the substrate 10 evolves as polishing progresses due tochanges in the thickness of the outermost layer, thus yielding asequence of time-varying spectra. Moreover, particular spectra areexhibited by particular thicknesses of the layer stack.

The computing device can process the signal to determine at least one ofwhether to adjust a polishing rate or an adjustment for the polishingrate. In particular, the computing device can execute logic thatdetermines, based a goodness of fit (as will be described in furtherdetail below), whether to adjust the polishing rate or the adjustmentfor the polishing rate.

In some implementations, the computing device can include a comparisonmodule that compares the measured spectra to multiple reference spectrato generate a sequence of best-match reference spectra, and determines agoodness of fit for the sequence of best-match reference spectra. Thecomputing device can also include an adjustment module that determinesan at least one of whether to adjust a polishing rate or an adjustmentfor the polishing rate, based on the goodness of fit.

As used herein, a reference spectrum is a predefined spectrum generatedprior to polishing of the substrate. A reference spectrum can have apre-defined association, i.e., defined prior to the polishing operation,with a value of a substrate property, such as a thickness of theoutermost layer. Alternatively or in addition, the reference spectrumcan have a pre-defined association with value representing a time in thepolishing process at which the spectrum is expected to appear, assumingthat the actual polishing rate follows an expected polishing rate.

A reference spectrum can be generated empirically, e.g., by measuringthe spectrum from a test substrate having a known layer thicknesses, orgenerated from theory. For example, to determine a reference spectrum, aspectrum of a “set-up” substrate with the same pattern as the productsubstrate can be measured pre-polish at a metrology station. A substrateproperty, e.g., the thickness of the outermost layer, can be also bemeasured pre-polish with the same metrology station or a differentmetrology station. The set-up substrate is then polished while spectraare collected. For each spectrum, a value is recorded representing thetime in the polishing process at which the spectrum was collected. Forexample, the value can be an elapsed time, or a number of platenrotations. The substrate can be overpolished, i.e., polished past adesired thickness, so that the spectrum of the light that reflected fromthe substrate when the target thickness is achieved can be obtained. Thespectrum and property, e.g., thickness of the outermost layer, of theset-up substrate can then be measured post-polish at a metrologystation.

Optionally, the set-up substrate can be removed periodically from thepolishing system, and its properties and/or spectrum measured at ametrology station, before being returned to polishing. A value can alsobe recorded representing the time in the polishing process at which thespectrum is measured at the metrology station.

The reference spectra are stored in a library. The reference spectra inthe library represent substrates with a variety of different thicknessesin the outer layer.

Multiple libraries can be created from different set-up substrates thatdiffer in characteristics other than the thickness of the outermostlayer, e.g., that differ in underlying layer thickness, underlying layerpattern, or outer or underlying layer composition.

The measured thicknesses and the collected spectra are used to select,from among the collected spectra, one or more spectra determined to beexhibited by the substrate when it had a thickness of interest. Inparticular, linear interpolation can be performed using the measured prepolish film thickness and post polish substrate thicknesses (or otherthicknesses measured at the metrology station) to determine the time andcorresponding spectrum exhibited when the target thickness was achieved.The spectrum or spectra determined to be exhibited when the targetthickness was achieved are designated to be the target spectrum ortarget spectra.

In addition, assuming a uniform polishing rate, a thickness of theoutermost layer can be calculated for each spectrum collected in-situusing linear interpolation between the measured pre polish filmthickness and post polish substrate thicknesses (or other thicknessesmeasured at the metrology station) based on the time at which thespectrum was collected. and time entries of the measured spectra.

In addition to being determined empirically, some or all of thereference spectra can be calculated from theory, e.g., using an opticalmodel of the substrate layers. For example, and optical model can beused to calculate a spectrum for a given outer layer thickness D. Avalue representing the time in the polishing process at which thespectrum would be collected can be calculated, e.g., by assuming thatthe outer layer is removed at a uniform polishing rate. For example, thetime Ts for a particular spectrum can be calculated simply by assuming astarting thickness D0 and uniform polishing rate R (Ts=(D0−D)/R). Asanother example, linear interpolation between measurement times T1, T2for the pre-polish and post-polish thicknesses D1, D2 (or otherthicknesses measured at the metrology station) based on the thickness Dused for the optical model can be performed (Ts=T2−T1*(D1−D)/(D1−D2)).

As used herein, a library of reference spectra is a collection ofreference spectra which represent substrates that share a property incommon (other than outer layer thickness). However, the property sharedin common in a single library may vary across multiple libraries ofreference spectra. For example, two different libraries can includereference spectra that represent substrates with two differentunderlying thicknesses.

Spectra for different libraries can be generated by polishing multiple“set-up” substrates with different substrate properties (e.g.,underlying layer thicknesses, or layer composition) and collectingspectra as discussed above; the spectra from one set-up substrate canprovide a first library and the spectra from another substrate with adifferent underlying layer thickness can provide a second library.Alternatively or in addition, reference spectra for different librariescan be calculated from theory, e.g., spectra for a first library can becalculated using the optical model with the underlying layer having afirst thickness, and spectra for a second library can be calculatedusing the optical model with the underlying layer having a different onethickness.

In some implementations, each reference spectrum is assigned an indexvalue. This index can be the value representing the time in thepolishing process at which the reference spectrum is expected to beobserved. The spectra can be indexed so that each spectrum in aparticular library has a unique index value. The indexing can beimplemented so that the index values are sequenced in an order in whichthe spectra were measured. An index value can be selected to changemonotonically, e.g., increase or decrease, as polishing progresses. Inparticular, the index values of the reference spectra can be selected sothat they form a linear function of time or number of platen rotations.For example, the index values can be proportional to a number of platenrotations. Thus, each index number can be a whole number, and the indexnumber can represent the expected platen rotation at which theassociated spectrum would appear.

The reference spectra and their associated indices can be stored in alibrary. The library can be implemented in memory of the computingdevice of the polishing apparatus. The index of the target spectrum canbe designated as a target index.

During polishing, an index trace can be generated for each library. Eachindex trace includes a sequence of indices that form the trace, eachparticular index of the sequence associated with a particular measuredspectrum. For the index trace of a given library, a particular index inthe sequence is generated by selecting the index of the referencespectrum from the given library that is the closest fit to a particularmeasured spectrum.

As shown in FIG. 3, the indices corresponding to each measured spectrumcan be plotted according to time or platen rotation. A polynomialfunction of known order, e.g., a first-order function (i.e., a line) isfit to the plotted index numbers, e.g., using robust line fitting. Wherethe line meets the target index defines the endpoint time or rotation.For example, a first-order function is fit to the data points as shownin FIG. 3.

Without being limited to any particular theory, some sequences ofindices can match the polynomial function better than others. Thegoodness of fit (e.g., a value that represents an amount the index tracediffers from the polynomial function) can be used to quantify thequality of the match. For example, FIG. 3 illustrates an example indextrace from a spectrographic monitoring system showing a good data fit.In comparison, FIG. 4 illustrates an example index trace from aspectrographic monitoring system showing a poorer data fit. Inparticular, the plotted index numbers in FIG. 4 have a greater amount ofdifference from an associated robust line, relatively, as compared tothe differences from an associated robust line and index traceassociated with plotted index numbers in FIG. 3. Quantifying andqualifying the amount of difference from the associated robust line willbe described in further detail below.

Because each reference spectrum (and its associated index) cancorrespond to a layer thickness of the substrate, the poorer fit canindicate that the quality of the signal used to generate the spectrameasurements may not be reliable. In some implementations, noise andother disturbances can affect the quality of the signal. For example,noise from the polishing apparatus can introduce variations into thesignal. In some implementations, variations in the polishing process canaffect the linearity of the index trace. For example, variations in theinitial thickness of the substrate layer, the slurry composition, andthe polishing pad window condition can result in variation in thereflected spectrum. As a result, measured spectra that are subject tothese variations can be matched to incorrect reference spectra. Thus,where a current polishing rate is calculated based on the line fittedsequence of indexes, the calculated current polishing rate may beunreliable, and a decision to adjust the polishing rate based on thecurrent polishing rate may also be unreliable. However, where thisimproper matching occurs, the fit of the line to the sequence of indexesshould be poorer. Thus, adjusting the polishing rate based on thegoodness of fit can avoid making an adjustment based on impropermatching of the measured spectra to reference spectra, which canincrease the accuracy and/or precision of the polishing process.

FIG. 5 is a flow diagram of an example process 500 for adjusting apolishing rate. A sequence of current spectra of reflected light from asubstrate is received (step 510). For example, the controller 60 canreceive a sequence of current spectra. Each current spectrum from thesequence of current spectra is compared (step 520) to a plurality ofreference spectra from a reference spectra library to generate asequence of best-match reference spectra. More generally, for eachcurrent spectrum, the reference spectrum that is the best match to thecurrent spectrum is determined. A goodness of fit is determined (step530) for the sequence of best-match reference spectra. For example, thesequence of best-match reference spectra can be fit to a line, and avariance from the line can be calculated. At least one of whether toadjust a polishing rate or an adjustment for the polishing rate isdetermined (step 540), based on the goodness of fit.

Determining an index trace's goodness of fit to a linear function caninclude determining the amount of difference between the index trace andthe associated robust line, e.g., the standard deviation, thecorrelation, or other measure of variance. For example, a goodness offit of zero can indicate that all of the index data points fit on theassociated robust line, e.g., the variance is zero. In someimplementations, the goodness of fit is determined by calculating a sumof squared differences between the index data points and the linearfunction. Other scales for the goodness of fit are possible.

In some implementations, if the goodness of fit is less than or equal toa threshold value (e.g., indicating the signal is reliable), then thepolishing rate can be adjusted. The threshold value can be auser-defined value. Alternatively, if the goodness of fit is greaterthan the threshold value (e.g., indicating the signal is unreliable),the polishing rate can be maintained.

In addition, an adjustment can be determined based on the goodness offit. In particular, a lower goodness of fit can indicate a better signalquality, and greater reliability of the spectra measurements. Therefore,in some implementations, the magnitude of the adjustment can beinversely proportional to the goodness of fit. A pressure in a carrierhead can be modified using the adjustment for the polishing rate. Insome implementations, a polishing endpoint can be determined, based onthe sequence of best-match reference spectra and the goodness of fit.

Although only one index trace is discussed above, the concept isapplicable to a plurality of index traces. In some implementations, thegoodness of fit of can be determined for each of a plurality of indextraces generated from a plurality of reference libraries. The conceptcan be used to determine which reference library to use in order tofurther determine whether to adjust a polishing rate and an adjustmentfor the polishing rate. For example, the system can use the referencelibrary that produces the best match to determine whether to adjust apolishing rate or an adjustment for the polishing rate. In particular,if at least one of the index traces has a goodness of fit that is lessthan or equal to a threshold value, then the polishing rate canadjusted, based on that particular goodness of fit. Alternatively, ifnone of the index traces has a goodness of fit that is less than orequal to the threshold value, then the polishing rate can be maintained.

FIG. 6 is a flow diagram of another example process 600 for adjusting apolishing rate. A sequence of current spectra of reflected light from asubstrate is received (step 610). Each current spectrum from thesequence of current spectra is compared (620) to a first plurality ofreference spectra from a first reference spectra library to generate afirst sequence of best-match reference spectra. Each current spectrumfrom the sequence of current spectra is compared (630) to a secondplurality of reference spectra from a second reference spectra libraryto generate a second sequence of best-match reference spectra. A firstgoodness of fit for the first sequence of best-match reference spectraand a second goodness of fit for the second sequence of best-matchreference spectra are determined (640). An adjustment for a polishingrate, based on at least one of the first goodness of fit and the secondgoodness of fit, is determined (650). For example, the first goodness offit can be compared to the second goodness of fit, and the goodness offit that indicates a better match can be used to determine the polishingrate.

Determining a difference between each of the current spectra and each ofthe reference spectra (step 520, 620, or 630) can include calculatingthe difference as a sum of differences in intensities over a range ofwavelengths. That is,

${Difference} = {\sum\limits_{\lambda = a}^{b}\; {{abs}\left( {{I_{current}(\lambda)} - {I_{reference}(\lambda)}} \right)}}$

where a and b are the lower limit and upper limit of the range ofwavelengths of a spectrum, respectively, and I_(current)(λ) andI_(reference)(λ) are the intensity of a current spectra and theintensity of the reference spectra for a given wavelength, respectively.Alternatively, the difference can be calculated as a mean square error,that is:

${Difference} = {\sum\limits_{\lambda = a}^{b}\; \left( {{I_{current}(\lambda)} - {I_{reference}(\lambda)}} \right)^{2}}$

Where there are multiple current spectra, a best match can be determinedbetween each of the current spectra and each of the reference spectra ofa given library. Each selected current spectra is compared against eachreference spectra. Given current spectra e, f, and g, and referencespectra E, F, and G, for example, a matching coefficient could becalculated for each of the following combinations of current andreference spectra: e and E, e and F, e and G,f and E,f and F,f and G, gand E, g and F, and g and G. Whichever matching coefficient indicatesthe best match, e.g., is the smallest, determines the referencespectrum, and thus the index.

Determining the goodness of fit of an index trace associated with aspectra library to the robust line associated with the spectra library(step 530 or 640) may include determining the least sum of squareddifferences between the data points comprising an index trace and therobust line fitted to the associated with the spectra library. Forexample, the least sum of squared differences between the data points asrepresented in FIG. 3 and FIG. 4 and their respective associated robustlines.

In some implementations, a spectrum is obtained at more than one radialposition of the substrate. For each spectra measurement, the radialposition on the substrate can be determined, and the spectrameasurements can be binned into zones based on their radial positions(e.g., radial zones). A substrate can have multiple zones, such as acenter zone, a middle zone and an edge zone. For example, on a 300 mmwafer, the center zone can extend from the center to a radius of 50 mm,the middle zone can extend from a radius of 50 mm to about 100 mm andthe edge can extend from about 100 mm to about 150 mm. In someimplementations, the substrate has more or fewer zones than the threementioned. The position from which the spectra is obtained can bedetermined, such as by using the method described in U.S. Pat. No.7,097,537, or as described in U.S. Pat. No. 7,018,271, incorporatedherein by reference for all purposes.

In some implementations, the sequence of current spectra can includespectra from a plurality of radial zones on the substrate. Generating asequence of best-match reference spectra can include generating aplurality of sequences of best-match reference spectra for each zone.The plurality of sequences of best-match reference spectra can be usedto determine at least one of whether to adjust a polishing rate or anadjustment for the polishing rate, based on the goodness of fit. Apolishing rate can be calculated for each particular zone based on thebest-match reference spectra from the sequence of current spectraassociated with the particular zone. In some implementations, a modifiedpolishing rate, for at least one zone, can be calculated such that theentire substrate reaches a target thickness at approximately the sametime. A modified polishing parameter can be calculated to provide themodified polishing rage, and the polishing rate can be adjusted bymodifying the polishing parameter.

The measured spectra from each zone (or, for each zone, an average ofspectra from within the zone obtained from a single sweep of the sensoracross the substrate) are compared to the reference spectra in thereference spectra library, as described above, and the correspondingindex number is determined for each zone from the comparison with thespectra library. The corresponding index numbers for each zone can beused to generate an index trace, and the index trace can be used todetermine a goodness of fit.

If the goodness of fit indicates that the spectra measurements arereliable (e.g., the goodness of fit is less than or equal to a thresholdvalue), the polishing rates in the zones can be adjusted using afeedback loop so that the final index number in each zone is equal tothe desired final index number. FIG. 7 is a flow diagram of an exampleprocess 700 for adjusting the polishing rate in zones to achieve adesired profile. The desired index number at the expected endpoint timeis determined for each zone on the substrate (step 702). Polishingcommences (step 704) and the substrate is optically monitored asdescribed above so that an index trace is determined for each zone onthe substrate (step 706). After an initial delay time, which allows thepolishing process to stabilize, the rate of change of the indexaccording to time is calculated (number of platen rotations may be usedas representative of the time) (step 708). The rate of change of theindex may be calculated simply as a difference in indexes at twodifferent times divided by the number of elapsed platen rotationsbetween the spectra measurements that generated the indexes at thedifferent times. The rate of change in the index number indicates thepolishing rate.

The rate of change of the index for each zone is used to extrapolate theindex trace to determine the index number that will be achieved at theexpected endpoint time for the associated zone (step 712). If at theexpected endpoint time, the desired index number will be passed or willnot yet been reached and the goodness of fit indicates that the spectrameasurements are reliable, the polishing rate can be adjusted upwardlyor downwardly, as required (step 720). If the desired index number isreached at the expected endpoint time, no adjustment may be required.More than one extrapolation and determination of whether an adjustmentshould be made can occur over the polishing sequence. Determiningwhether an adjustment in the polishing rate is appropriate can includedetermining whether the goodness of fit indicates that the spectrameasurements are reliable, and determining whether the desired indexnumber will be achieved when the polishing endpoint occurs ordetermining that the final index falls within an acceptable range fromthe desired final index number.

In some implementations, the expected endpoint time is determined forone zone, such as the center zone. The polishing rates within the otherzones are then adjusted, if appropriate, to achieve their desiredendpoints at the same time as the expected endpoint time for theselected zone, e.g., the center zone. The polishing rates can beadjusted, such as by increasing or decreasing the pressure in acorresponding zone in the carrier head. In some carrier heads, such asthe carrier head described in U.S. Publication No. 2005-0211377, thecarrier head has adjustable pressure zones. The change in polishing ratecan be assumed to be directly proportional to the change in pressure,e.g., a simple Prestonian model. Additionally, a control model forpolishing the substrates can be developed that takes into account theinfluences of platen or head rotational speed, second order effects ofdifferent head pressure combinations, the polishing temperature, slurryflow, or other parameters that affect the polishing rate.

Referring to FIG. 8, if a particular profile is desired, such as auniform thickness across the surface of the substrate, the slope of thepolishing rate, as indicated by the change in index numbers according totime, can be monitored and the polishing rate adjusted if the goodnessof fit of the index trace indicates that the spectra measurements arereliable. After a polishing stabilizing period 805, a spectrum isobtained at the center zone 810, at the edge zone 815 and in between ata middle zone 820. Here, the zones are circular or annular zones. Eachspectrum is correlated to its respective index. This process is repeatedover a number of platen rotations, or over time, and the polishing rateat each of the center zone 810, middle zone 820 and edge zone 815 isdetermined. The polishing rate is indicated by the slope of the linethat is obtained by plotting the index 830 (y-axis) according to thenumber of rotations 835 (x-axis). If any of the rates is calculated tobe faster or slower than the others, the rate in the zone can beadjusted if the goodness of fit of the index trace indicates that thespectra measurements are reliable. Here, the adjustment is based on theendpoint C_(E) of the center zone 810. For some implementations, if thepolishing rates are within an acceptable margin, no adjustment need bemade. An approximate polish end point EDP is known from polishingsimilar substrates with similar polishing parameters or from using thedifference method described above. At a first polishing time T₁ duringthe polishing process, the rate of polishing at the middle zone 820 isdecreased and the rate of polishing at the edge zone is increased.Without adjusting the polishing rate at the middle zone 820, the middlezone would be polished faster than the rest of the substrate, beingpolished at an overpolish rate of M_(A). Without adjusting the polishingrate at T₁ for the edge zone 815, the edge zone 815 would beunderpolished at a rate of E_(u).

At a subsequent time (T₂) during the polishing process, the rates canagain be adjusted, if appropriate. The goal in this polishing process isto end polishing when the substrate has a flat surface, or an oxidelayer across the surface that is relatively even. One way of determiningthe amount to adjust the rate of polishing is to adjust the rates sothat the index of each of the center, middle and edge zones are equal atthe approximate polish end point EDP. Thus, the polishing rate at theedge zone needs adjusting while the center and middle zones are polishedat the same rate as prior to T₂. If the EDP is approximate, polishingcan be stopped when the index at each zone is in the desired location,that is, when each location has the same index.

During the polishing process, it is preferred to only make changes inthe polishing rates a few times, such as four, three, two or only onetime. The adjustment can be made near the beginning, at the middle ortoward the end of the polishing process. Associating the spectra with anindex number creates a linear comparison for polishing at each of thezones and can simplify calculations required to determine how to controlthe polishing process and obviate complex software or processing steps.

A method that can be applied during the polishing process is to limitthe portion of the library that is searched for matching spectra. Thelibrary typically includes a wider range of spectra than will beobtained while polishing a substrate. The wider range accounts forspectra obtained from a thicker starting outermost layer and spectraobtained after overpolishing. During substrate polishing, the librarysearching is limited to a predetermined range of library spectra. Insome embodiments, the current rotational index N of a substrate beingpolished is determined. N can be determined by searching all of thelibrary spectra. For the spectra obtained during a subsequent rotation,the library is searched within a range of freedom of N. That is, ifduring one rotation the index number is found to be N, during asubsequent rotation which is X rotations later, where the freedom is Y,the range that will be searched from (N+X)−Y to (N+X)+Y. For example, ifat the first polishing rotation of a substrate, the matching index isfound to be 8 and the freedom is selected to be 5, for spectra obtainedduring the second rotation, only spectra corresponding to index numbers9±5 are examined for a match. When this method is applied, the samemethod can be independently applied to all of the libraries currentlybeing used in the endpoint detection process.

Embodiments of the invention and all of the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructural means disclosed in this specification and structuralequivalents thereof, or in combinations of them. Embodiments of theinvention can be implemented as one or more computer program products,i.e., one or more computer programs tangibly embodied in an informationcarrier, e.g., in a machine-readable storage media or in a propagatedsignal, for execution by, or to control the operation of, dataprocessing apparatus, e.g., a programmable processor, a computer, ormultiple processors or computers. A computer program (also known as aprogram, software, software application, or code) can be written in anyform of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program does notnecessarily correspond to a file. A program can be stored in a portionof a file that holds other programs or data, in a single file dedicatedto the program in question, or in multiple coordinated files (e.g.,files that store one or more modules, sub-programs, or portions ofcode). A computer program can be deployed to be executed on one computeror on multiple computers at one site or distributed across multiplesites and interconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierhead, or both can move to provide relative motion between the polishingsurface and the substrate. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems, e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly. Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and substrate can beheld in a vertical orientation or some other orientation.

Particular embodiments of the invention have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results.

1. A computer-implemented method, comprising: receiving a sequence ofcurrent spectra of reflected light from a substrate; comparing eachcurrent spectrum from the sequence of current spectra to a plurality ofreference spectra from a reference spectra library to generate asequence of best-match reference spectra; determining a goodness of fitfor the sequence of best-match reference spectra; and determining atleast one of whether to adjust a polishing rate or an adjustment for thepolishing rate, based on the goodness of fit.
 2. The method of claim 1,wherein the goodness of fit is less than or equal to a threshold valueand the polishing rate is adjusted.
 3. The method of claim 1, whereinthe goodness of fit is greater than a threshold value and the polishingrate is maintained.
 4. The method of claim 1, further comprising:modifying a pressure in a carrier head using the adjustment for thepolishing rate.
 5. The method of claim 1, wherein the sequence ofcurrent spectra includes spectra from a plurality of radial zones on thesubstrate, and generating a sequence of best-match reference spectraincludes generating a plurality of sequences of best-match referencespectra, each zone having an associated sequence of best-match referencespectra.
 6. The method of claim 5, wherein determining an adjustmentincludes calculating a polishing rate for each particular zone based onthe best-match reference spectra from the sequence of current spectraassociated with the particular zone.
 7. The method of claim 5, whereindetermining an adjustment includes calculating a modified polishing ratefor at least one zone such that the entire substrate reaches a targetthickness at approximately the same time.
 8. The method of claim 7,wherein determining an adjustment includes calculating a modifiedpolishing parameter to provide the modified polishing rate, and furthercomprising adjusting the polishing rate by modifying the polishingparameter
 9. The method of claim 1, wherein determining the goodness offit includes: determining a sequence of indices from the referencespectra library that corresponds to the sequence of best-match referencespectra; and fitting the sequence of indices to a function.
 10. Themethod of claim 9, wherein the function produces a line.
 11. The methodof claim 9, wherein the goodness of fit is based on an amount that thesequence of indices varies from the function.
 12. The method of claim11, further comprising: calculating the amount using a sum of squareddifferences between the sequence of indices and the function.
 13. Themethod of claim 1, further comprising: determining a polishing endpointbased on the sequence of best-match reference spectra and the goodnessof fit.
 14. A computer-implemented method comprising: receiving asequence of current spectra of reflected light from a substrate;comparing each current spectrum from the sequence of current spectra toa first plurality of reference spectra from a first reference spectralibrary to generate a first sequence of best-match reference spectra;comparing each current spectrum from the sequence of current spectra toa second plurality of reference spectra from a second reference spectralibrary to generate a second sequence of best-match reference spectra;determining a first goodness of fit for the first sequence of best-matchreference spectra and a second goodness of fit for the second sequenceof best-match reference spectra; and determining at least one of whetherto adjust a polishing rate or an adjustment for the polishing rate,based on at least one of the first goodness of fit and the secondgoodness of fit.
 15. The method of claim 14, wherein at least one of thefirst goodness of fit and the second goodness of fit is less than orequal to a threshold value, and the polishing rate is adjusted.
 16. Themethod of claim 14, wherein determining the first goodness of fitincludes: determining a first sequence of indices from the firstreference spectra library that corresponds to the first sequence ofbest-match reference spectra; and fitting the first sequence of indicesto a first function.
 17. The method of claim 16, wherein determining thesecond goodness of fit includes: determining a second sequence ofindices from the second reference spectra library that corresponds tothe second sequence of best-match reference spectra; and fitting thesecond sequence of indices to a second function.
 18. The method of claim17, wherein the first function and the second function each produce aline.
 19. The method of claim 17, wherein the first goodness of fit isbased on a first amount that the first sequence of indices varies fromthe first function, and the second goodness of fit is based on a secondamount that the second sequence of indices varies from the secondfunction.
 20. The method of claim 19, further comprising: calculatingthe first amount using a sum of squared differences between the firstsequence of indices and the first function; and calculating the secondamount using a sum of squared differences between the second sequence ofindices and the second function.
 21. The method of claim 14, furthercomprising: determining a polishing endpoint based on at least one of:the first sequence of best-match reference spectra and the firstgoodness of fit; and the second sequence of best-match reference spectraand the second goodness of fit.
 22. A controller comprising: acomparison module that compares each current spectrum from a sequence ofcurrent spectra of reflected light from a substrate to a plurality ofreference spectra from one or more reference spectra libraries togenerate a sequence of best-match reference spectra, and determines agoodness of fit for the sequence of best-match reference spectra; and anadjustment module that determines at least one of whether to adjust apolishing rate or an adjustment for the polishing rate, based on thegoodness of fit.
 23. A system comprising: a light source; a detectorthat detects light reflected from the substrate and determines asequence of current spectra from the light; and a controller thatincludes: a comparison module that compares each current spectrum fromthe sequence of current spectra to a plurality of reference spectra fromone or more reference spectra libraries to generate a sequence ofbest-match reference spectra, and determines a goodness of fit for thesequence of best-match reference spectra; and an adjustment module thatdetermines at least one of whether to adjust a polishing rate or anadjustment for the polishing rate, based on the goodness of fit.
 24. Acomputer program product, encoded on a tangible program carrier,operable to cause data processing apparatus to perform operationscomprising: receiving a sequence of current spectra of reflected lightfrom a substrate; comparing each current spectrum from the sequence ofcurrent spectra to a plurality of reference spectra from a referencespectra library to generate a sequence of best-match reference spectra;determining a goodness of fit for the sequence of best-match referencespectra; and determining at least one of whether to adjust a polishingrate or an adjustment for the polishing rate, based on the goodness offit.